Hydrostatic pressure effect on hydrophobic hydration and pairwise hydrophobic interaction of methane

Abstract

At room temperature, the Ben-Naim standard hydration Gibbs energy of methane is a positive quantity that increases markedly with hydrostatic pressure [M. S. Moghaddam and H. S. Chan, J. Chem. Phys. 126, 114507 (2007)]. This finding is rationalized by showing that the magnitude of the reversible work to create a suitable cavity in water increases with pressure due to both the increase in the volume packing density of water and the contribution of the pressure-volume work. According to the present approach, at room temperature, the Gibbs energy of the contact-minimum configuration of two methane molecules is a negative quantity that increases in magnitude with hydrostatic pressure. This result is not in line with the results of several computer simulation studies [T. Ghosh, A. E. Garcia, and S. Garde, J. Am. Chem. Soc. 123, 10997-11003 (2001)], and emerges because pairwise association causes a decrease in solvent-excluded volume that produces a gain of configurational/translational entropy of water molecules, whose magnitude increases with the volume packing density of the liquid phase.